Fevers are in theory useful but in practice dont make that much of a difference to the course of most illness.

Most fevers can be left untreated, as it is rare for the body to generate a fever high enough to be harmful.

However, fever is very uncomfortable, and can interfere with other things that are important to healing - hydration being the big one - lots of folks, especially kids, will throw up and have trouble drinking when they have a fever, so bringing the fever down can help with a vital part of staying alive.

Treat fever if the symptoms of the fever itself are troublesome, otherwise generally fine to let it do its thing.

The quick test will be a point of care test (like a glucose meter but for hb) - they'll then collect more blood to check much more detailed tests - these will give good indications of why the hb is so low - blood count is pretty helpful for leukaemia as you'd find way too many of one type of blood cell. Most common is low iron, also possible low b12. Then there's other things but basically boils down to not making enough blood, or losing too much.

People panicking breathe too fast, and breathe out more co2 than your body is used to. This makes you feel weird (light-headedness, fingers tingling, that sort of thing).

The idea is that re breathing the air you just breathed out should slow the co2 loss and make you feel better. My understanding of why we stopped doing it is its just a lot more effective to get the person to slow their breathing by talking them through it, and the paper bag thing never practically worked that well (while the theory is sound, in practice you have to strike a reasonably fine balance in having a rebreathing circuit open enough to allow plenty of oxygen in but still retain co2, and that's just really tricky).

Epipens can be dangerous, even when administered correctly.

People need to know that even though the symptoms should improve when they take the epipen they still need to go to hospital because other treatment and often additional adrenalin may be required- epipen is kinda a first aid move.

Diagnosis of who needs an epipen can be complex - its not always clear to people if what they had was anaphylaxis or not (severe episodes are generally fairly identifiable, but milder episodes can be tricky - a combo of rash and vomiting might need someone to have access to an epipen even though thats not what most people think of when they think anaphylaxis).